The present invention relates to a novel color-retaining excess-temperature exposure indicator. More particularly, but not exclusively, the invention relates to a novel historical temperature indicator capable of providing a permanent visual indication of past exposure of the indicator to an elevated temperature exceeding a threshold temperature. The invention also relates to such a temperature exposure indicator which is particularly, but not exclusively, useful for monitoring the temperature exposure experience of host products that have high temperature limits with time requirements, for example, fish, meat and other products. The invention also relates to processes for manufacturing such indicators. It is recognized that certain perishable products, especially sensitive foodstuffs such as fresh eggs and fish may have their quality or safety compromised by relatively brief exposures to a temperature in excess of a predetermined threshold. For example, fresh meat and poultry should usually not be exposed for significant time periods to temperatures in excess of 4.4° C. (40° F.). Fresh eggs should not be exposed to temperatures in excess of 7.2° C. (45° F.). In some cases, government regulations may mandate that certain thresholds not be exceeded.
There is accordingly a need for simple, low-cost indicators which can be associated with or attached to a perishable host product to indicate the past occurrence of an excess temperature exposure to a temperature above a threshold.
Prusik et al. U.S. Pat. No. 5,709,472 (“Prusik '472” hereinafter) and its divisional, U.S. Pat. No. 6,042,264 describe and claim a time-temperature indicator label for measuring the length of time to which a product has been exposed to a temperature above a pre-determined temperature. The label is a composite of a plurality of layers that can be adhered at its underside to a product container. The label includes a printable surface layer, a longitudinal wicking strip that is adhered underneath the surface layer substantially at the opposite extremities only of the wicking strip and a lower substrate layer forming an envelope with the surface layer. A heat-fusible substance, which melts and flows above a pre-determined temperature, is applied on the surface of the wicking strip contiguous to at least one of the ends of the wicking member. When the heat-fusible substance is exposed to a temperature above the pre-determined temperature, the heat fusible substance flows along the length of the wicking member. The surface layer is provided with a sight window at an intermediate location over the wicking member through which the progress of flow on the wicking member is observed. This progress can signal the exposure of a product that has exceeded a permissible exposure of time above a pre-determined temperature
U.S. Pat. No. 3,954,011 (Manske); U.S. Pat. No. 5,120,137 (Ou-Yang) and U.S. Pat. No. 5,368,905 (Ohno) disclose further temperature exposure indicators. The disclosures of these patents are discussed in Prusik '472.
The foregoing description of background art may include insights, discoveries, understandings or disclosures, or associations together of disclosures, that were not known to the relevant art prior to the present invention but which were provided by the invention. Some such contributions of the invention may have been specifically pointed out herein, whereas other such contributions of the invention will be apparent from their context. Merely because a document may have been cited here, no admission is made that the field of the document, which may be quite different from that of the invention, is analogous to the field or fields of the present invention.